Field of the Invention
The invention relates to devices and methods for the acquisition of mass spectra of ions separated by their mobility.
Description of the Related Art
In document U.S. Pat. No. 7,838,826 B1 (M. A. Park, 2008), a relatively small ion mobility spectrometer is presented which is termed “trapped ion mobility spectrometer” (TIMS). The length of the essential mobility separation unit, the separator tunnel, amounts to about five centimeters only, not counting additional entrance and exit funnels. The separation of ions according to their mobilities is based upon a gas flow in the cylindrical separator tunnel which drives the ions from an ion source in an accumulation phase against a counter-acting electric DC field barrier while the ions are radially confined by a quadrupolar RF field. After shutting down the delivery of further ions, a scan phase starts, in which the electric DC field barrier is steadily decreased. Ions are driven in the scan phase by the gas flow over the decreasing electric DC field barrier, thereby releasing, successively, ions from low mobilities to higher and higher mobilities from being trapped by the barrier. The ions can be detected in an ion detector, resulting in a mobility spectrum.
FIG. 1 schematically shows a preferred design and operation of a trapping ion mobility spectrometer as described in U.S. Pat. No. 7,838,826 B1. The tube-like separator tunnel (11) between entrance funnel (10) and exit funnel (12) amount to only 48 millimeters in length; the inner diameter amounts to eight millimeters. The ion mobility separator tunnel (11) consists of a series of segmented diaphragms with quadrant electrodes (1), (2), (3), and (4), for generating a quadrupolar RF field. Ions (6) from a source (not shown) are introduced by capillary (8) together with a gas stream (7) into a first vacuum chamber. A repeller plate (9) directs the ions into the funnel (10); the gas flow (14) drives the ions into the separator tunnel (11). In the bottom part of FIG. 1, electric field profiles E(z) along the z-axis are shown for three phases of operation: In the accumulation phase (A) ions are blown by the gas flow (16) against the rising edge of the electric field profile between z locations (20) and (23). In a trap phase (B) of only one to two milliseconds, the inflow of ions is stopped and ions assume their equilibrium position on the rising edge according to their mobility. The steadily decreasing profile voltage in the scan phase (C) releases ions in the order of increasing ion mobility over the plateau of the electric field between locations (23) and (24) and through the exit funnel towards an ion detector. Particularly, the ions may be measured by a mass spectrometer, e.g. a time-of-flight mass spectrometer, resulting in a two-dimensional mass-mobility spectrum. Unlike many other trials to build small ion mobility spectrometers, the device by M. A. Park has already achieved ion mobility resolutions up to Rmob=250, a very high ion mobility resolution never achieved by conventional mobility spectrometers.
There is still a need for devices and methods operating with highest utility rates (duty cycle) of the ions generated in an ion source of a mass spectrometer, thereby reducing the restriction of the mobility resolution, in particular with an electrospray ion source coupled to liquid chromatography for analyzing complex samples in the field of bottom-up proteomics.